XML Element of XML Element of XML Element (and so on...)

Question

Here is my XML data (shorted, in a file myFile):
<LiveData>
<Voltage>
<Total>
<VoltageNow>1052</VoltageNow>
</Total>
</Voltage>
<Power>
<Total>
<PowerNow>345</PowerNow>
</Total>
</LiveData>
I would like to get the VoltageNow and PowerNow values, but the following script does not work:
tell application "System Events"
set elem1 to XML element 1 of XML file myFile /*<LiveData>
set power to XML elements of elem1 whose name is "Power"
set total to XML elements of power whose name is "Total"
set powernow to value of XML elements of total whose name is "PowerNow"
end tell
Error: System Events got an error: Can’t make {} into type string.
BTW, I don't want to use XML Element X because the order may change.
What is the correct method to traverse these elements to obtain values?

Answer 1

AppleScript can handle the strings, but the System Events XML Suite doesn't work with text strings - it deals with the XML objects, which have name and value properties. The handler can be modified to show the names of an element, but it is just designed to work with the XML Suite.
The contents of an XML file is just text, so you can forgo System Events and just use regular text manipulation statements, or use another tool such as the Satimage XMLLib osax, which is much more robust and does include commands to display the XML object as a string.
By the way, my original handler was designed and tested in Leopard, but I discovered a Snow Leopard bug that alters the way it returns array/list contents. The following script has changes that return the functionality, in addition to a dialog handler that will display lists:
<pre style="
font-family: Monaco, 'Courier New', Courier, monospace;
font-size: 10px;
font-weight: normal;
margin: 0px;
padding: 5px;
border: 1px solid #000000;
width: 720px; height: 340px;
color: #000000;
background-color: #DAFFB6;
overflow: auto;"
title="this text can be pasted into the Script Editor">
on run -- example
set XMLFile to (choose file without invisibles)
show("element value:", (getXMLElement from {XMLFile, "LiveData", "Voltage", "Total", "VoltageNow"}))
end run
to show(label, something)
show an alert for something, formatting lists and records
parameters - label [text]: the alert text
something [mixed]: an item to put in the alert message
returns nothing
if something is not in {null, missing value} then
try -- show the message
if (class of something) is in {list, record} then (something as number) -- force an error
display alert label message (something as text)
on error errorMessage -- lists and records don't format very well, so cheat
set {here, there} to {(offset of "{" in errorMessage), -(offset of "}" in (reverse of text items of errorMessage) as text)}
display alert label message text here thru there of errorMessage
end try
else
display alert label message "*null or missing value*"
end if
end show
to getXMLElement from XMLItems
get the specified element from a simple XML structure by name or index
    the number of items is not fixed, but must be at least 2 (the XML file and an XML element)
  parameters: XMLItems [list] -
item 1 [text]: the XML file path (Finder or POSIX)
item 2 [mixed]: the XML element name or index (names are case sensitive)
item(s) 3+ [mixed]: sub item(s)
returns [list]: value of the element
try
if (count XMLItems) is less than 2 then error "getXMLElement handler:  item list contains too few items"
tell application "System Events"
set theXMLData to contents of XML file ((the first item of XMLItems) as text)
-- get text of theXMLData
set theXMLElement to XML element 1 of theXMLData -- start at the root element
repeat with anItem in rest of XMLItems -- add on the sub items
set anItem to contents of anItem
try
set anItem to anItem as integer -- index number?
end try
set theXMLElement to (get XML element anItem of theXMLElement)
end repeat
try -- return the element(s)
-- return name of XML elements of theXMLElement
set theResult to value of theXMLElement -- SL built-in result variable does not get set
if theResult is missing value then error -- SL does not error on empty or missing value
return theResult
on error errorMessage number errorNumber -- the element is empty, or a list of items
log errorMessage
return value of XML elements of theXMLElement
end try
end tell
on error errorMessage number errorNumber
log errorMessage
-- return null
error "getXMLElement handler:  element not found (" & errorNumber & ")"
end try
end getXMLElement</pre>